656 



ECOLOGY AND EVOLUTION 



and inorganic chemistry, except for the 

 phenomenon of momentum, and present 

 conditions alone determine a process 

 (Lewis, 1946). In biology, reaction to pres- 

 ent conditions may be determined through 

 learned behavior from past experiences. 

 Also, reaction to present conditions may be 

 determined through selective sorting in past 

 environments, and such selection eflFects are 

 augmented by the repetition of the environ- 

 mental factors and conditions. If orderly 

 continuation and repetition of habitat con- 

 ditions did not occur, adaptive evolution 

 would be weak or absent. Some philoso- 

 phers think that scientists base their method 

 on a faith in the orderliness of nature. 

 There seems to be much evidence that order 

 and continuity are objectively real phenom- 

 ena that have been discovered and par- 

 tially analyzed by scientific method. 



The "directiveness" of organic processes 

 (often referred to as purposive or teleologi- 

 cal) has its explanation in differential sur- 

 vival of autocatalytic systems under re- 

 peated natural conditions (see also p. 639). 

 The organism must be viewed in its evolu- 

 tionary perspective to resolve these ancient 

 philosophical problems (see Russell, 1945, 

 for a discussion largely avoiding evolution- 

 ary analysis; see also Hutchinson, 1948). 



^X COMPETITION AND SELECTION 



Competition (p. 395) may increase se- 

 lection pressure, but it is not the only fac- 

 tor operating in selection (see Elton, 1930, 

 p. 39). Some of the most obvious and 

 widespread adaptations, such as tracheae 

 and lungs, are adjustments to factors so 

 pervasive as to involve no competition for 

 a limited supplv. Aside from a few habitats, 

 oxygen occurs in sufficient abundance for 

 all animals and plants, and the evolution of 

 breathing adaptations is possibly through 

 survival of the fit rather than only through 

 survival of the fittest. 



Competition or rivalry between individ- 

 uals for limited necessities is a conse- 

 quence of overproduction. Since Darwin's 

 day, ample evidence as to the amount of 

 absolute potential reproductive capacity be- 

 yond the possibility of survival has been 

 gathered (p. 272). The partial potential re- 

 productive capacity is the maximum possi- 

 ble under a given set of restricted condi- 

 tions, and here we encounter selective fac- 

 tors operating upon what genetic variability 



may be present. Competition greatly in- 

 creases the action of selection upon genetic 

 differences and creates more subtle survival 

 distinctions. Mortality (pp. 273, 368) 

 commonly is the result of competition and 

 is often a selective mortality of genetic ef- 

 fects. Competition may be between cooper- 

 ative systems, and in this way may result in 

 the e^'olution of increased cooperation and 

 decreased destructive competition (Alice, 

 1940). 



The role of competition may be highly 

 important, even in adaptation to a physical 

 factor. During the drought of 1933 to 1940 

 in the prairies of Nebraska, Colorado, and 

 Kansas, the cactus, Opuntia macrorrhiza, 

 became abundant in former grasslands, but 

 in the moister years of 1941 to 1943, it 

 was largely replaced by grasses (Timmons, 

 1941-42; Albertson and Weaver, 1944). 

 Dense growths of vegetation competed 

 with the cactus for soil water and light, 

 and by transpiration increased the humid- 

 ity. Large numbers of insects were able to 

 kill the cactus under moist conditions, 

 though they had not been very harmful to 

 it during the drought period. Without com- 

 petition, some vegetation changes of this 

 type might take place, but competition 

 probably plays an important role (see T. 

 Park (1948) for an experimental analysis of 

 competing beetle populations). 



Competition is conceived by some to be 

 greater between individuals of the same 

 species than between individuals of differ- 

 ent species, and greater between taxonomi- 

 cally closely related species or ecologically 

 equivalent species within the same asso- 

 ciation than between more adaptively diver- 

 gent species. It is difficult to find data from 

 which a valid generalization on these 

 points can be made. So far as can be 

 judged, intraspecies competition is more 

 severe in some cases and interspecies com- 

 petition in others (Hutchinson, 1948). 



Genetic differentials in competition ef- 

 fects among litter mates within the mam- 

 malian uterus have been demonstrated (see 

 p. 403). Wright and Eaton (1929) re- 

 ported that one inbred family of guinea 

 pigs showed the highest percentages born 

 alive in litters of three, with litters of two 

 and four close behind, while there is a 

 marked drop in litters of both one and five. 

 In another inbred family, the highest per- 

 centage born alive was in litters of one, 



